This invention relates to catalysts and in particular to catalysts suitable for use for hydrogenation, especially the hydrogenation of oils and fats.
Oils and fats are often either partially or fully hydrogenated in a batch slurry process by suspending a particulate nickel catalyst in the oil or fat and feeding hydrogen thereto while heating the mixture, typically to a temperature in the range 80 to 250xc2x0 C., possibly under pressure, e.g. at a pressure of up to 30 bar abs. For partial hydrogenation, the pressure is usually under 10 bar abs., for example 2 to 4 bar abs. For oil or fat hydrogenation, the catalyst should have a high activity so that the desired degree of hydrogenation can be achieved in a short time and/or a small amount of nickel can be employed. The catalyst should also exhibit a good selectivity in the case of partial hydrogenation so that over-hydrogenation of the oils and fats is minimised. Furthermore it is desirable that the residual catalyst can be readily filtered from the hydrogenated oil or fat and that the catalyst show good refuse properties.
Catalysts often employed for this process are nickel on a support of e.g. alumina and are characterised by, inter alia, a high nickel surface area per gram of nickel. Typical catalysts having a high nickel content are described in EP 0 168 091, wherein the catalyst is made by precipitation of a nickel compound and then a soluble aluminium compound is added to the slurry of the precipitated nickel compound while the precipitate is maturing, i.e. ageing. After reduction of the resultant catalyst precursor, the reduced catalyst typically has a nickel surface area of the order of 90 to 150 m2 per g of total nickel. The catalysts have a nickel/aluminium atomic ratio in the range 2 to 10. Reduced catalysts having a nickel/aluminium atomic ratio above 2, in which at least 70% by weight of the total nickel has been reduced to elemental nickel, have a total nickel content of more than about 66% by weight.
Nickel/alumina hydrogenation catalysts, having a total nickel content of 5 to 40% by weight, but also having a high nickel surface area, made by a different route are described in U.S. Pat. No. 4,490,480. In the process of this latter reference, a nickel ammine complex, particularly a nickel ammine carbonate, is heated in the presence a transition alumina: this results in the precipitation of a nickel compound, such as nickel hydroxide or basic nickel carbonate, intimately associated with the alumina. In this latter process, an alumina powder may be slurried with a solution of the nickel complex, or shaped units, such as spheres or cylindrical extrudates, typically having a minimum dimension above about 1.5 mm, formed from the alumina are impregnated with a solution of the nickel complex. While catalysts having a nickel surface area over 130 m2 per g total nickel, and indeed in some cases above 200 m2 per g total nickel, are described, such high surface area products are all made by the aforesaid impregnation route using shaped alumina units: the catalysts made by slurrying alumina powder with the nickel complex have nickel surface areas significantly below 130 m2 per g total nickel. While catalysts made using the preformed, shaped alumina units are of utility in fixed bed hydrogenation processes, they are unsuitable for the aforesaid batch slurry hydrogenation process as their size renders them liable to settling out from the slurry, and also, when used for partial hydrogenation, they tend to give over hydrogenation of the fats and oils. The aforementioned U.S. Pat. No. 4,490,480 indicates that catalysts suitable for batch slurry hydrogenation may be made by grinding high nickel surface area catalysts made by the aforesaid impregnation route using shaped alumina units. However the production of such catalysts by such a technique involves additional processing steps of forming the alumina into the shaped units and the subsequent comminution step.
Catalysts made directly from an alumina powder of 60-70 xcexcm size containing 18-28% by weight of nickel and having a nickel surface area of up to 123 m2 per g of nickel are also described in the aforesaid U.S. Pat. No. 4,490,480. However we have found that such materials had a relatively poor activity for the hydrogenation of oils.
We have now found that nickel/alumina catalysts having a high activity and/or good selectivity may be made by the aforesaid process employing a slurry of the alumina powder if an alumina powder having a small particle size is employed. Surprisingly, despite the use of a small particle size alumina, the catalysts are readily filtered from the hydrogenated fat or oil.
It has been proposed in GB 926 235 to make hydrogenation catalysts by this route using kieselguhr as the support. However, we have found that catalysts made using small particle size kieselguhr, as opposed to transition alumina, do not exhibit high nickel surface areas.
Accordingly we provide a method of making a nickel/alumina catalyst containing 5 to 75% by weight of total nickel comprising slurrying a transition alumina powder having a surface-weighted mean diameter D[3,2] in the range 1 xcexcm to 20 xcexcm with an aqueous solution of a nickel ammine complex, heating the slurry to cause the nickel amine complex to decompose with the deposition of an insoluble nickel compound, filtering the solid residue from the aqueous medium, drying and, optionally after calcining the solid residue, reducing the solid residue.
By the term total nickel, we mean the amount of nickel whether present in elemental or combined form. Generally however at least 70% by weight of the total nickel in the reduced catalyst will be in the elemental state.
The term surface-weighted mean diameter D[3,2], otherwise termed the Sauter mean diameter, is defined by M. Alderliesten in the paper xe2x80x9cA Nomenclature for Mean Particle Diametersxe2x80x9d; Anal. Proc., vol 21, May 1984, pages 167-172, and is calculated from the particle size analysis which may conveniently be effected by laser diffraction for example using a Malvern Mastersizer.
The transition alumina may be of the gamma-alumina group, for example a eta-alumina or chi-alumina. These materials may be formed by calcination of aluminium hydroxides at 400-750xc2x0 C. and generally have a BET surface area in the range 150-400 m2/g. Alternatively, the transition alumina may be of the delta-alumina group which includes the high temperature forms such as delta- and theta-aluminas which may be formed by heating a gamma group alumina to a temperature above about 800xc2x0 C. The delta-group aluminas generally have a BET surface area in the range 50-150 m2/g. The transition aluminas contain less than 0.5 mol of water per mole of Al2O3, the actual amount of water depending on the temperature to which they have been heated. The alumina should be porous, preferably having a pore volume of at least 0.2 ml/g, particularly in the range 0.3 to 1 ml/g.
It is preferred that the small particle size alumina has a relatively large average pore diameter as the use of such aluminas appears to give catalysts of particularly good selectivity. Preferred aluminas have an average pore diameter of at least 12 nm, particularly in the range 15 to 30 nm. [By the term average pore diameter we mean 4 times the pore volume as measured from the desorption branch of the nitrogen physisorption isotherm at 0.98 relative pressure divided by the BET surface area]. During the production of the catalyst, nickel compounds are deposited in the pores of the alumina, and so the average pore diameter of the catalyst will be less than that of the alumina employed, and decreases as the proportion of nickel increases. It is preferred that the reduced catalysts have an average pore diameter of at least 10 nm, preferably above 15 nm and particularly in the range 15 to 25 nm.
On the other hand, irrespective of the nickel content of the catalyst, the particle size of the catalyst is essentially the same as the particle size of the transition alumina, and so the catalysts generally have a surface-weighted mean diameter D[3,2] in the range 1 to 20 xcexcm, and is preferably less than 10 xcexcm, particularly less than 8 xcexcm.
The catalysts of the invention contain 5 to 75% by weight of total nickel, preferably below 70% by weight total nickel. Catalysts containing up to about 55%, preferably 5 to 45%, by weight total nickel, typically have a nickel surface area above 130, preferably above 150, more preferably above 180, and in particular above 200, m2 per gram total nickel.
Accordingly the present invention also provides a particulate nickel/transition alumina catalyst containing 5 to 55% by weight of total nickel, having a nickel surface area of at least 130 m2 per gram of total nickel, and a surface-weighted mean diameter D[3,2] in the range 1 xcexcm to 20 xcexcm.
The nickel surface area may be determined as described in xe2x80x9cPhysical and Chemical Aspects of Adsorbents and Catalystsxe2x80x9d, edited by B. G. Linsen, Academic Press, 1970 London and New York, page 494 and 495, and is a measure of the surface area of the reduced, i.e. elemental, nickel in the catalyst.
We have found that in general, the nickel surface area of catalysts made by the process of the invention tends to decrease as the nickel content increases. However we have also found that catalysts made using large pore size aluminas and containing relatively large amounts of nickel are surprisingly active and selective even though they may not have such a high nickel surface area. Thus useful catalysts containing at least 20% by weight total nickel having an average pore diameter above 10 nm and a nickel surface area above 110 m2/g total nickel may be made using large pore aluminas.
Accordingly the present invention also provides a particulate nickel/transition alumina catalyst containing 20 to 75% by weight of total nickel, having a nickel surface area of at least 110 m2 per gram of total nickel, a surface-weighted mean diameter D[3,2] in the range 1 xcexcm to 20 xcexcm, and an average pore diameter of at least 10 nm, preferably above 12 nm, and particularly in the range 15 to 25 nm.
Catalysts containing at least 20% by weight total nickel having a nickel surface area as low as 80 m2/g total nickel appear to have good activity and selectivity provided that the average pore diameter is above 15 nm.
Accordingly the present invention also provides a particulate nickel/transition alumina catalyst containing 20 to 75% by weight of total nickel, having a nickel surface area of at least 80 m2 per gram of total nickel, a surface-weighted mean diameter D[3,2] in the range 1 xcexcm to 20 xcexcm, and an average pore diameter of at least 15 nm.
The catalysts may be made by slurrying the transition alumina powder with the appropriate amount of an aqueous solution of a nickel ammine complex, e.g. the product of dissolving basic nickel carbonate in a solution of ammonium carbonate in aqueous ammonium hydroxide, to give a product of the desired nickel content. The solution of the nickel ammine complex preferably has a pH in the range 9 to 10.5. The slurry is then heated, e.g. to a temperature in the range 60 to 100xc2x0 C., to cause the nickel amine complex to decompose with the evolution of ammonia and carbon dioxide and to deposit an insoluble nickel compound, e.g. basic nickel carbonate on the surface, and in the pores, of the transition alumina. The alumina carrying the deposited nickel compound is then filtered from the aqueous medium and dried. It may then be calcined in air, e.g. at a temperature in the range 250 to 450xc2x0 C., to decompose the deposited nickel compound to nickel oxide. Upon reduction of the nickel oxide, the high nickel surface area is generated. Alternatively the deposited nickel compound may be directly reduced, i.e. without the need for a calcination step. The reduction, whether or not a preliminary calcination step is employed, may be effected by heating to a temperature in the range 250 to 450xc2x0 C. in the presence of hydrogen.
As indicated above, the catalysts are of particular utility for the hydrogenation of fats and oils, such as fish oil, soybean oil, rapeseed oil, and sunflower oil. Alternatively the catalysts may be used for other hydrogenation reactions such as the hydrogenation of olefinic compounds, e.g. waxes, nitro or nitrile compounds, e.g. the conversion of nitrobenzene to aniline or the conversion of nitrites to amines. They may also be used for the hydrogenation of paraffin waxes to remove traces of unsaturation therein.
As indicated above, in such a hydrogenation process, the requisite amount of catalyst is suspended in a charge of the oil or fat and the mixture heated, possibly under pressure, while hydrogen is introduced, e.g. sparged through the mixture. Conveniently the catalyst is charged to the hydrogenation vessel as a concentrate of the catalyst particles dispersed in a suitable carrier medium, e.g. hardened soybean oil. Preferably the amount of catalyst in said concentrate is such that the concentrate has a total nickel content of 5 to 30%, preferably 10 to 25% by weight.
Alternatively, in some cases the reduction may be effected in situ. Thus a precursor comprising the transition alumina and the unreduced nickel compound, e.g. oxide, possibly as a concentrate, i.e. dispersed in a carrier as aforesaid, may be charged to the hydrogenation reactor with the material to be hydrogenated and the mixture heated while hydrogen is sparged through the mixture.
Accordingly we also provide a catalyst precursor comprising a transition alumina and a reducible nickel compound, which when reduced with hydrogen at a temperature in the range 250 to 450xc2x0 C. gives a particulate catalyst containing 5 to 55% by weight of total nickel, having a nickel surface area of at least 130 m2 per gram of total nickel, and a surface-weighted mean diameter D[3,2] of 1 xcexcm to 20 xcexcm, preferably less than 10 xcexcm.
We also provide a catalyst precursor comprising a transition alumina and a reducible nickel compound, which when reduced with hydrogen at a temperature in the range 250 to 450xc2x0 C. gives a particulate catalyst containing 20 to 75% by weight of total nickel, having a nickel surface area of at least 80 m2 per gram of total nickel, and a surface-weighted mean diameter D[3,2] of 1 xcexcm to 20 xcexcm, preferably less than 10 xcexcm, and an average pore diameter above 10 nm.
The invention is illustrated by the following examples in which, unless otherwise specified, all percentages and parts per million (ppm) are by weight. The nickel surface areas are determined as described in the aforesaid xe2x80x9cPhysical and Chemical Aspects of Adsorbents and Catalystsxe2x80x9d, edited by B. G. Linsen, Academic Press, 1970 London and New York, at pages 494-495 using a reduction time of 1 hour.